This invention was supported in part by research grant number DE09322-01 to the American Dental Association Health Foundation from the National Institute of Dental Research.
This invention relates to the field of shielding healthy tissues from radiation damage during radiotherapy for malignant conditions. It also relates particularly to the field of such shielding where the tissues to be shielded are of irregular conformation and/or associated with metallic restorations or prostheses, as in the mouth.
Approximately 30,000 people were diagnosed with some form of oral cancer in 1985, accounting for more than 3% of all patients with cancer and 3% of cancer related deaths (American Cancer Society, 1985). Treatment of oral malignancies is a great challenge to radiotherapists because of their potential curability.
Treatment of head and neck tumors by electron, x-, or gamma-ray teletherapy is a well established and highly successful modality. S. Benak, F. Buschke, and M. Galanta, "Treatment of Carcinoma of the Oral Cavity," Radiology 96, 137-143 (1970), T. L. Phillips, and S. Benak, "Radiation Modalities in Treatment of Cancer of the Oral Cavity," J. Prosthet. Dent. 27, 413 (1972); J. A. Toljanic, and V. W. Saunders, "Radiation Therapy and Management of the Irradiated Patient," J. Prosthet. Dent. 52, 852 (1984). The ease of access, localization of malignancies, and relatively high responsiveness of these tumors to radiation leads to encouraging treatment prognoses.
One of the major complications of head and neck radiotherapy is the post-irradiation damage to healthy tissues in front of, adjacent to, or beyond the treated tumors. This latent radiation damage to nonmalignant tissues can range in severity from slight post-treatment discomfort to life-threatening necrosis. Manifestations of radiation damage include dry mouth (xerostomia), loss of taste, changes in oral microflora and salivary chemistry, erythema and ulceration of oral mucosa, glossitis and edema of the tongue, moniliasis of the lips, salivary gland dysfunction and edema, dysphagia, muscle fibrosis, and osteonecrosis. J. Beumer, S. Silverman, and S. B. Benak, "Hard and Soft Tissue Necrosis Following Radiation Therapy for Oral Cancer," J. Prosthet. Dent. 27:640-644 (1972); J. Beumer, T. R. Curtis, and R. Harrison, "Radiation Therapy of the Oral Cavity: Sequelae and Management," Part 1, Head and Neck Surgery, 1:301-312 (1979); D. L. Larson, "Management of Complications of Radiotherapy of the Head and Neck," Surgical Clinics of North America 66:169-182 (1986); S. Driezen, L. R. Brown, S. Handler, and B. M. Levy, "Radiation-Induced Xerostomia in Cancer Patients, Effect on Salivary and Serum Electrolytes," Cancer 38, 273-278 (1976); S. Driezen, T. E. Daly, J. B. Drane, and L. R. Brown, "Oral Complications of Cancer Radiotherapy," Postgrad. Med. 61, 85-92 (1977); cited by I. L. Shannon, "Management of Head and Neck Irradiated Patients," Adv. Physiol Sci. Vol. 28, Saliva and Salivation (1980). Diminished salivary function is a very common post-irradiation condition which often leads to accelerated tooth decay or "radiation caries". J. Beumer, S. Silverman, and S. B. Benak, "Hard and Soft Tissue Necroses Following Radiation Therapy for Oral Cancer," J. Prosthet. Dent. 27:640-644 (1972); J. Beumer, T. R. Curtis, and R. Harrison, "Radiation Therapy of the Oral Cavity: Sequelae and Management," Part 1, Head and Neck Surgery 1:301-408 (1979), C. Fernandez, S. Master, B. Sarosh, and K Turner, "Efficacy of Radiation Protection Prosthesis in Controlling Radiation Induced Xerostomia," J. Indian Dent. Assoc, 56:371-378 (1984).
The severity of the above side effects on normal tissues has been reduced by a number of techniques including selection of the radiation source to have the least effect on normal surrounding or overlying tissues, careful positioning and collimation of the source beam, and shielding.
In treatment of head and neck lesions with high-intensity radiation (teletherapy), an important aspect of the protection of healthy tissues has thus been manufacture and application of an individually customized prosthetic appliance, which is designed, modelled, and formed into a custom-made metal on plastic shield. Specifically, in treating lesions of the skin or oral tissues with electrons, photons, x-, or gamma-rays, shields and stents containing cast forms made from metals or alloys of high atomic density elements have been used to protect surrounding tissues.
The fabrication of these appliances is a multi-step procedure often requiring the cooperative efforts of the radiotherapist and the dentist/prosthodontist. First, impressions are made of the intra- or extraoral treatment site and a plaster model of the tissues fabricated from these. A wax replica of the shielding prosthesis is fabricated on the plaster model and this is cast in polymerized acrylic by a lost-wax method to form the working stent. This stent is tried for fit and then hollowed out in the appropriate region for incorporation of a metal liner, which serves as the customized radiation shield. Molten lead or a low-temperature-melting alloy such as Lipowitz metal (50% bismuth, 26.7% lead, 3.3% tin, and 10% cadmium) is then poured and formed into the working polyacrylic stent, to form the shielding appliance, leaving a window exposing only the tissue being irradiated, where appropriate. After cooling, the metal casting is covered with an additional layer of polyacrylic. The completed appliance is then polished and adjusted to the final fit. Textbook of Radiotherapy, edited by G. H. Fletcher, second ed. (Philadelphia, Lea and Febiger, 1973).